1. Field of the Invention
The present invention generally relates to image processing techniques for controlling the distortion of an inter-unit boundary in the case of decompressing, unit by unit, a compressed image divided into predetermined units, and more particularly to an image decoding device, an image processing apparatus, and an image processing method suitable for suppressing a tile boundary distortion on an image decompressed after being compressed according to JPEG2000. Such an image decoding device may be employed in personal computers (PCs), personal digital assistants (PDAs), mobile phones, and digital cameras, for example, which are apparatuses for handling images such as Internet-based images, medical images, and satellite communication images. Such an image processing apparatus may be a personal computer, a PDA, a mobile phone, or a digital camera, for example, using such an image decoding device.
2. Description of the Related Art
In many cases, image data is temporarily compressed to be stored or transmitted, and the compressed image data is decompressed and output to be processed whenever or wherever necessary. For image data compression, an image compression method such as JPEG, which divides an image into predetermined division units called blocks and encodes the image using discrete cosine transform for each block, is widely used. Such an image compression method has a problem in that when the compressed image is decompressed, the decompressed image includes block boundary distortion. There are well-known methods that detect the block boundary distortion and perform low-pass filtering on the pixels of the block boundaries of the decompressed image in order to make the block boundary distortion less conspicuous. Such methods are disclosed in Japanese Laid-Open Patent Applications No. 5-316361 and No. 9-307855 and Japanese Patent No. 2839987, for instance.
Recently, improvements in image input and output technologies have greatly increased demand for high-definition images. In the case of digital cameras as image input apparatuses, for instance, high-performance charge coupled devices (CCDs) having 3,000,000 pixels or more have been reduced in price to be widely used in digital cameras in a popular price range. It is expected that products employing CCDs having 5,000,000 pixels or more will be commercially available in the near future. It is expected that this trend toward an increasing number of pixels will continue for a while.
On the other hand, there have also been remarkable developments in the high-definition property and significant progress in the price reduction of image output apparatuses and image display apparatuses such as hard-copy apparatuses including laser printers, ink-jet printers, and sublimation-type printers, and soft-copy apparatuses including flat panel displays made of CRTs, liquid crystal displays (LCDs), and plasma display panels (PDPs).
Due to the introduction of these high-performance, inexpensive image input and output apparatuses to the market, high-definition images have become popular. As a result, it is expected that there will be an increasing demand for high-definition images in various fields in the future. Actually, the developments in technologies related to PCs and networks including the Internet have accelerated such trends at an increasing rate. Particularly in recent years, mobile equipment such as mobile phones and notebook personal computers has become so popular that opportunities to transmit or receive high-definition images anywhere through communication means have increased rapidly.
It seems inevitable that, with these background trends, demand for improvement in the performance and multi-functioning of image compression and/or decompression technologies will become stronger in the future so that processing of high-definition images can be facilitated.
Therefore, in recent years, a new image compression method called JPEG2000, which can restore with high-quality an image compressed at a high compression rate, has been standardized as one of image compression techniques satisfying such demand. According to JPEG2000, by dividing an image into rectangular regions called tiles, compression and decompression can be performed on the image with a small memory capacity. That is, each individual tile serves as a basic unit in performing compression and decompression processes, so that the tiles can be subjected to the compression and decompression processes independent of one another.